(a) Field of the Invention
The present invention relates to a polymer blend electrolyte membrane comprising an inorganic polymer having polydimethylsiloxane as a main chain, which has a pore structure at both ends formed by a condensation reaction between 3-aminopropyltriethoxysilane and tetraethylorthosilicate, wherein phosphoric acid is chemically linked to an amino group of the pore structure; and a proton-conducting polymer having a cation exchange group at the side chain thereof, as well as a manufacturing method thereof. Generally, proton-conducting electrolyte membranes have significantly reduced ion conductivity at high temperatures. However, proton-conducting electrolyte membranes have advantages in terms of efficiency and cost, and thus there remains a need to develop an electrolyte membrane which has excellent ion conductivity, even at high temperatures. Accordingly, the present invention aims to provide a polymer blend electrolyte membrane for use at high temperature and a manufacturing method thereof.
(b) Background Art
A fuel cell is an electrochemical device that converts the chemical energy of hydrogen and oxygen directly into electrical energy, and is a new electricity generation technology that continuously produces electricity by supplying hydrogen and oxygen to anode and cathode electrodes.
With respect to the general properties of fuel cells, heat is also generated in a process of producing electricity by the electrochemical reaction of fuel, making it possible to achieve high-efficiency electricity generation at a total efficiency of more than 80%, and the fuel cell has an efficiency higher than that of existing thermal power generation, making it possible to save fuel for electricity generation and to perform co-generation. In addition, the fuel cell is a pollution-free energy technology, in which the emissions of nitrogen oxides and CO2 are about 1/38 and ⅓, respectively, compared to those of coal burning thermal power generation. The level of noise pollution is also very low, so that pollutants are not substantially discharged.
In addition, because fuel cell modules can be manufactured, construction of the fuel cell plant can be shortened, an increase or decrease in the equipment capacity of the fuel cell plant is possible, and the site selection of the fuel cell plant is easy. Thus, because the fuel cell can be placed in urban areas or buildings, it can economically supply energy. Also, because the fuel cell can employ various fuels, including, but not only limited to, natural gas, city gas, naphtha, methanol and waste gases, it can substitute for existing thermal power generation and can be applied in power plants for distributed generation, co-generation power plants, power sources for pollution-free automobiles, and the like.
Recently, due to environmental problems and the exhaustion of energy sources, and in order to use fuel cell automobiles in practice, there has been a need for high-performance fuel cells having high energy efficiency and that can be operated at high temperatures and, at the same time, that are reliable. In addition, in order to increase the efficiency of such fuel cells, the development of polymer membranes, which can be used at high temperatures, has also been required. Fuel cells are largely classified into molten carbonate fuel cells (MCFCs) operating at high temperatures (500-700° C.), phosphoric acid fuel cells (PAFCs) operating at about 200° C., alkaline fuel cells (AFCs) operating in the range from room temperature to about 100° C., and polymer electrolyte fuel cells.
Among these fuel cells, the polymer electrolyte fuel cells are an example of a future clean energy source capable of substituting for fossil energy and have high output density and energy conversion efficiency. Also, the polymer electrolyte fuel cells can be operated at room temperature and can be miniaturized and closed, and thus they can be used in a wide range of applications, including pollution-free automobiles, residential power generation systems, mobile communication systems, medical devices, military equipment, and equipment for space applications.
Such polymer electrolyte fuel cells can largely be classified into proton exchange membrane fuel cell (PEMFCs), which use hydrogen gas as a fuel, and direct methanol fuel cells (DMFCs), which use liquid methanol, supplied directly to the anode, as a fuel.
The proton exchange membrane fuel cell (PEMFC) is a power production system that produces direct current electricity from an electrochemical reaction of hydrogen with oxygen, and the general structure of PEMFC is shown in exemplary FIG. 1. The PEMFC has a structure in which a proton-conducting polymer membrane is interposed between an anode and a cathode. Specifically, PEMFC may comprise: a proton-conducting polymer membrane, which has a thickness of 50-200 μm and made of a solid polymer electrolyte; an anode and a cathode (hereinafter, the cathode and anode will be commonly referred to as “gas diffusion electrodes”), which suitably comprise the respective support layers for the supply of reaction gas, and the respective catalyst layers in which oxidation/reduction reactions occur; and a carbon plate, which has grooves for gas injection and functions as a current collector. The catalyst layers in the gas diffusion electrodes of PEMFC are suitably formed on the support layers, respectively, in which the support layers are made of carbon cloth or carbon paper, and the surfaces thereof are treated such that reaction gas, water, which is transferred to the proton-conducting polymer membrane, and water resulting from the reactions, are easily passed.
In PEMFC having the above-described structure, the reaction gas hydrogen is supplied, while an oxidation reaction occurs in the anode to convert hydrogen molecules to hydrogen ions and electrons, and the converted hydrogen ions are transferred to the cathode through the proton-conducting polymer membrane. In the cathode, a reduction reaction, in which oxygen molecules become oxygen ions by receiving electrons, occurs, and the produced oxygen ions are converted to water molecules by reacting with the hydrogen ions that are transferred from the anode.
The proton-conducting polymer membrane functions to transfer protons, generated in the anode, to the cathode. In order to obtain a high output (i.e., high current density) in PEMFC, the conduction of protons needs to be performed in a sufficient amount at a high rate. Accordingly, the performance of the proton-conducting polymer membrane is important in determining the performance of PEMFC. In addition to its function to conduct protons, the proton-conducting polymer membrane functions as an insulating film to electrically insulate the anode and the cathode, and also functions as a fuel barrier film for preventing a fuel, supplied to the anode, from leaking to the cathode.
One example of a main proton conducting membrane, which is used in PEFC at present, is a fluororesin-based membrane having a perfluoroalkylene as a main skeleton and partly having a sulfonic acid group at the end of perfluorovinylether side chain. Known examples of such sulfonated fluororesin-based membranes include, but are not limited to, Nafion (trade name) (produced by E.I. Dupont de Nemours), Flemion (trade name) film (produced by Asahi Glass KK), Aciplex (trade name) film (produced by Asahi Chemical Industry Co.), etc. These fluororesin-based membranes have chemical structures, shown in the following formula I and Table 1.
TABLE 1Fluororesin-based membranes (structural parameters forformula I) produced by various manufacturers)[Formula I] StructuralTradeEquivalentThicknessparametersManufacturernameweight(□)m = 1;DupontNafion 1201200260x = 5-13.5;Nafion 1171100175n = 2;Nafion 1151100125y = 1Nafion 1121100 80M = 0.1,AsahiFlemion-T1000120n = 1.5GlassFlemion-S1000 80Flemion-R1000 50M = 0;AsahiAciplex-S1000- 25-n = 2-5;Chemical1200100x = 1.5-14
These exemplary fluororesin-based membranes are considered to have a glass transition temperature (Tg) in the vicinity of 130° C. under suitably wet conditions where the fuel cell is used. In the vicinity of this temperature, so-called creep occurs.
As a result, the protonic conduction structure in the membrane changes, making it impossible to attain stable protonic conduction performance. Furthermore, the membrane is denatured to a swollen state which, after prolonged exposure to high temperature, becomes jelly-like and thus can easily break, leading to failure of the fuel cell.
For the aforementioned reasons, the current highest temperature at which the fuel cell can be used stably over an extended period of time is normally 80° C.
A fuel cell employs chemical reaction in principle, and thus exhibits a higher energy efficiency when operated at higher temperatures. Accordingly, when considered on the basis of the same electricity output, a device which can be operated at higher temperatures can be suitably reduced more in size and weight. Furthermore, when the fuel cell is operated at high temperatures, its exhaust heat can be utilized as well, allowing cogeneration (combined supply of heat and electricity) that enhances the total energy efficiency.
Accordingly, it is considered that the operating temperature of a fuel cell is somewhat higher, normally 100° C., particularly preferably 120° C. or more (see, for example, Korean Patent Registration No. 10-0701549).
When the polymer electrolyte fuel cell is operated at temperatures higher than 100° C., the activity of the electrode catalyst and the reaction rate of the electrode can suitably increase, and thus the fuel cell performance can be improved with a reduced amount of the catalyst.
Further, a decrease in the amount of use of an expensive platinum catalyst can lead to a suitable decrease in the cost of the fuel cell system. Furthermore, a few ppm of hydrocarbon contained in reformed hydrogen fuel is oxidized to carbon monoxide by a catalytic reaction on the electrode surface, and the generated carbon monoxide is adsorbed on the surface of the platinum catalyst to poison the catalyst. The adsorption of carbon monoxide onto the catalyst is an exothermic reaction, and thus when the fuel cell is operated at suitably high temperatures, the performance of the fuel cell can be stably improved, because catalyst poisoning can be suitably reduced, even when reformed hydrogen gas containing a small amount of hydrocarbon is used. Preferably, when the fuel cell can be operated with external pressurization, an external pressurizing device and humidifying device becomes simple or unnecessary, thus providing advantages in terms of the optimization of the entire system and costs.
In the case of direct fuel cells (e.g., DMFC) which directly use fuels other than hydrogen, studies focused on efficiently extracting protons and electrons from fuels have been conducted. However, the improvement in the fuel barrier property of the proton-conducting polymer membrane, and operation at a high temperature at which a catalyst effectively functions, are considered to be important factors necessary to obtain a sufficient output.
Accordingly, although it may be considered desirable that PEFC is operated at higher temperatures, the heat resistance of the proton conducting membrane is up to 80□ as previously mentioned and the operating temperature of the fuel cell, too, is thus limited to 80□ at present.
The reaction occurring during the operation of a fuel cell is an exothermic reaction, and when a fuel cell is operated, the temperature in PEFC rises spontaneously. However, since Nafion, which is a representative proton conducting membrane that is used at present, has only heat resistance up to about 80□, it is necessary that PEFC be cooled so that the temperature does not reach 80□. Cooling is normally carried out by a water cooling method, and the separator portion of PEFC is devised for such cooling. When such a cooling unit is employed, the entire system of PEFC has an increased size and weight, making it impossible to make sufficient use of the original characteristics of PEFC, which are small size and light weight.
In particular, when the limit of operation temperature is 80° C., a water cooling system, which is the simplest cooling system, can make effective cooling difficult. If operation at 100□ or more is made possible, effective cooling can be made by releasing the evaporation heat of water, and when water is circulated, the amount of water to be used in cooling can be drastically reduced, thus making it possible to attain the reduction of size and weight of the device.
In particular, in cases where PEFC is used as an energy source for vehicles, the comparison of the system involving the temperature control to 80° C. with the system involving the temperature control to 100° C. or more shows that the volume of radiator and cooling water can be drastically reduced, and it has thus been desired to provide PEFC which can operate at 100° C. or more, i.e., a proton conducting membrane having a heat resistance of 100° C. or more.
As mentioned herein, although PEFC has been required to operate at high temperatures, that is, proton conducting membranes are required to have high temperature resistance for electricity generating efficiency, cogeneration efficiency, cost, resources and cooling efficiency, no proton conducting membranes having both sufficient protonic conductivity and heat resistance exist. For this reason, various polymer materials and organic/inorganic composite materials, which have excellent electrochemical properties and thermal properties and, at the same time, can overcome the above-described problems, have been proposed for use.
A representative example of these heat-resistant proton conducting materials is a heat-resistant aromatic polymer material, Examples of such a heat-resistant aromatic polymer material include polybenzimidazoles, polyethersulfones and polyether ether ketones, etc. However, these aromatic polymers have a problem in that they are difficult to manufacture in the form of a membrane because they are very hard, and thus are difficult to dissolve.
Furthermore, these aromatic polymer materials exhibit deteriorated heat resistance or hot water resistance similar to the fluororesin-based membranes and can be dissolved in hot water in some cases. Moreover, when water is present, the entire membrane tends to swell similar to the fluororesin-based membrane, and due to the change of the size of the membrane, stress is applied to the junction of the membrane-electrode assembly, making it very likely that the membrane and the electrode can be exfoliated at the junction, or that the membrane can be broken, and thus there may be a reduction of strength of the membrane due to swelling that can cause membrane destruction. Furthermore, since these aromatic polymer materials are polymer compounds which stay rigid when dried, the membrane may undergo destruction during the formation of the membrane-electrode assembly.
In order to solve these problems, methods for introducing these electrolytes into a porous resin have been studied (see U.S. Pat. No. 6,242,135). In this case, the film strength and dimensional stability can be improved, but the proton conducting membrane used remains the same and heat stability is not improved. (Korean Patent Registration No. 10-0701549).
Moreover, studies on composites with inorganic materials (e.g., silica), which having high water absorption capability, have been conducted, but the inorganic materials are not conductive or the conductivity is lower than that of organic materials.
Published literature relating to proton-conducting polymers, which use such inorganic materials to improve the physical and chemical properties thereof, have been described as follows. U.S. Pat. No. 5,283,310 discloses a polymer, which contains —O—Si(WX)—O—Si(YZ)—R1— as a fundamental structure and forms an inorganic-inorganic copolymer network. US Patent Publication No. 2004-146766 discloses an organic-inorganic conductive polymer, which has a silane group as a skeleton and contains a nitro group, which is linked to silicon by an alkyl group. Korean Patent Laid-Open Publication No. 2005-19667 discloses a network-type polymer, in which ethylene oxide is linked to the side chain of a siloxane polymer, such that the siloxane polymer can stably form a network, thus improving the mechanical properties, chemical stability and ion conductivity of the polymer. In addition, Korean Patent Laid-Open Publication No. 1999-82205 discloses a macropolymer, which is formed of a polymer having a silicon-oxygen main chain and thus has high electrical conductivity at room temperature.
Although phosphoric acid shows excellent proton-conducting properties at high temperature and room temperature, it is advantageous in that phosphoric acid is leached with water, when it is humidified. For this reason, the operating temperature of a phosphoric acid fuel cell is limited to temperatures higher than the boiling point of water and, in addition, time and power are consumed to increase the operating temperature. The present inventors have found that, when phosphoric acid is chemically linked to an inorganic polymer, which has more desirable electrochemical properties and thermal and mechanical stabilities, a proton-conducting polymer membrane can be manufactured.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that is already known to a person skilled in that art.